Face making and Loop Cutting
I suspect that the problem is that you are trying to make a loop cut through a face which does not have four edges [a "quad", short for "quadrilateral"]. When a face has fewer than, or more than four sides, the intelligence in the loop cut does not know where you want the loop cut routed.

Sometimes, the loop cut will form partway around the faces. When this occurs, at the point where the loop cut fails to complete, you can manually continue the loop cut by selecting the edge, or vertex through which you want the loop cut to continue, and manually delete the fact, and add new faces using the conventional methods.